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Home Design

I recently built my new home and tried to incorporate all the ebergy efficient options. The house is built on an ICF foundation, I have WallTite Eco foam on the rim joists, walls and entire roof deck and my windows are triple pane, 2 compartment gas filled fixed or casement style. Heting/air conditioning is provided by 7 tons of GeoThermal system. When the initial plans where evaluated I was told I could heat the house with light bulbs and would require very little airconditioning. So far I am a bit disappointed and baffled by the performance of the house. We have recently experienced some fairly warm weather and the geoThermal system in cooling mode has kept the house at the set 73F. What baffles me is in the evenings and night time when the outside temperature drops as low as 64F the house temperature will still rise above 73 and call for air conditioning. it is as if the house is acting as a heat dam and releasing heat back into the house at night. This may seem like a good thing but causes the air conditionig to run excessively or else we must monitor the situation and turn off the AC and open the windows in the evening. This isn't such a bad thing but seems to defeat the idea of building an energy efficient home and relyying on the technology to keep us comfortable. I guess what I am wondering, is this behaviour to be expected. Is the foam in the walls absorbing heat during the day and releasing it back into the house at night? any explanation would be appreciated. I should note that we do have about 85 sq. ft. of glass on the West exposure. Thanks.

What you are experiencing isn't uncommon in well sealed high mass homes. Most people just either pay for the electricity at night or open a window if the humidity isn't too high. If you run the A/C a little at night it won't add that much to your power bill. In commercial there are things call "economizers" that bring in cool air from outside during times when it's cooler outside than inside. For residential the payback time for economizers is too long to make sense.

how many light bulbs are you running at night?
how are your cooking appliances exhausting?
how many people living in the house?
what type of geothermal system? loop piping? vertical piping? water source from a well?
clothes dryer ventilation? water heating?

The TRUE highest cost system is the system not installed properly...
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Dwane Johnson
The A/C repairman

how many light bulbs are you running at night?
how are your cooking appliances exhausting?
how many people living in the house?
what type of geothermal system? loop piping? vertical piping? water source from a well?
clothes dryer ventilation? water heating?

I assume you realize heating by light bulbs was tongue-in-cheek
The kitchen exhaust is not yet installed but cooking generates minimal heat with just two of us old folks in the house; we seldom use the oven in summer and the cook top is magnetic induction.
geothermal is ground loop and the dryer vents directly to the outside. Thanks.

There are two primary issues at play when you're trying to evaluate the efficiency of your home as built.

1. The 'fly-wheel effect' in the cooling industry is a term used to explain that it takes time for a home to both heat up and then to cool down when outdoor temperatures change. For example, in morning, the sun is low in the sky for the first part of the day but is adding Btu's to the outside shell of the home. As the day wears on and outdoor temperatures increase, the sun rises in the sky, the outdoor temps go higher and again, all that is being pushed into the exterior of your home. But you have insulation to slow the effect of those influences and so it's about 3:00 pm before they fully affect the interior. So your cooling system can 'loaf' along in the early morning, gradually increasing run minutes per hour until sometime in mid to late afternoon when the heat gain is highest. Now that your house exterior walls are fully heated, they'll continue to radiate that heat into the home until early evening. As the sun and outdoor temperature both recede, the walls and insulation of your home are still loaded with heat and will continue to raise the temperature for a few more hours, requiring operation of the cooling system well into the evening.

2. The second issue at play is the very insulation you put into the home. The assumption is that the heating and cooling equipment is smaller than would be needed had you not taken pains to make the house efficient. That means the cost to operate the equipment is also lower. But here's the important point. The insulation can only SLOW the transfer of heat from warmer to cooler. Heat always flows from warm to cold but the tighter the home, the slower that heat transfers back to the outdoors when the outdoor temperature is lower than the indoor temperature. In an ideal world, you've have high insulation values when the outdoor temperature is higher than indoors and low insulation values when the indoor temperature is higher but then only in the summer. In winter, you'd want the opposite. You want the insulation to keep the heat indoors. So in effect, the insulation works for you in one situation and against you in the other. Once the house is all warmed up, the insulation is just doing what you wanted it to do, that is, keeping the heat in!

If YOU want change, YOU have to first change.

If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

Check you electronics, depending how many you have plugged in, you are getting a fair a mount of heat from them. Also, lights can tend to put off a fair amount of heat, so that a well sealed and insulated home tends to increase in temp. What type of water heater do you have, and where is it located.

I appreciate the good explanation. This is kinda what I thought was happening but not what i expected. I guess i need to evaluate the cost of simply letting the airconditioning go 24x7 in hot weather vs. switching it off and opening the windows when the temp dips in the evening and then switching back in the morning before the heat builds up. Again, thanks for the explanation.

How large is this hous and where are you located? 7 ton on a well sealed ICF home is a LOT, even if you lived in Houston or South Florida unless it's over 7000sqft. As others mentioned, the concerte provides thermal mass. Thsi means that you interior never "experiences" the leak tempratures of the day, so the equipment can be undersized against normal load calculations. Other than solar gain through windows and air leakage and ventilation, it will tend to behave as though the temperature stays equal to the average of hte outdoor temperature. Therefore I'd expect it to run at a similar duty cycle as late as 3-5AM before the structure equalizes to the outdoor conditions.

This is a good thing, because if the equipemnt was properly sized, it will have very long run times throughout the night keeping humidity more constant.

Opening windows at night in most climates is usually counterproductive. In many cases the dewpoints sharply rise after sunset and don't drop until early morning, so you'll bring in a lot of humidity. Opening windwos doesn;t pay off unitl your in "economizing" type of weather, usually in April and October in most areas where overnight temps are under 55F.

Actually in a large ICF home, using commerical equipment with econoimizers wouldn't be a terrible idea. You could also dehumidify using hot gas reheat as well. Even better, if your in a summer location, might have been to use a chilled water system.

OP said house is built on an ICF foundation, which suggests the house has a basement. I'm also under the impression the walls are conventionally framed with external foam board sheathing, and perhaps the attic is also insulated at the roof deck.

What we did not receive is how large this house is. Seven tons may sound large even if my interpretation of the construction techniques is correct, but we have no disclosure from the OP how large the house is, or where it is located.

Skippedover nailed it, however. The house is performing exactly as designed...to hold heat in! Hence the need for a/c as exterior temperatures fall below room temperature setpoint. During the day, the driver of heat is very strong - the sun - so even great insulation will allow some amount of heat to enter the building, if slowly. At night, the energy forces between indoors and outdoors, at say 74 indoors and 65 outdoors, is not nearly as strong, so the transfer of heat outward into the cool night air is VERY slow. Hence the need for a/c or open windows.

OP, sounds like you live in a cold climate and your house was designed accordingly. Do you find the house easy to heat in winter?

Electricity makes refrigeration happen.

Refrigeration makes the HVAC psychrometric process happen.

HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.

Thganks for your input You guys all seem to agree that the performance of the house is as should be expected and your explanations make sense.

For your info; the house is 2200 sq. ft. Bungalo with a full ICF basement. The house is stickbuilt (2x6) with 3 1/2" of medium density sprayfoam in the wall cavities. The entire roof deck is covered with 5" of spray foam on the underside and the Rim joist are spraued with about 5" of foam. Only 3 tons of the geothermal contributes to AC. The other 4 tons is water-to-water heat (should have made that clear).

Beenthere, you touched on a good point about the lights and electronics. That's what prompted my first statement that I could heat the house with lightbulbs. Incidently I have 2 connected 60 galllon water heaters. The first tank is heated by the Geothermal desupraheater and feeds the 2nd. tank that is electric.

The bottom line is, all of you Pros on the forumn offered a good explanation that confirms what I thought was happening and I feel a bit better knowing that this is normal behaviour. Again, thanks to everyone for your comments.

Even with your concern, I bet that house is a lot easier to keep cool and warm than an equivalent sized and built house that did not use the extensive insulation and air sealing efforts you undertook with your place.

Electricity makes refrigeration happen.

Refrigeration makes the HVAC psychrometric process happen.

HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.

I doubt that the internal loads would be enough to create a significant runtime in the evening / night.

It seems like most OPs have a real reluctance to provide their location.

It seems that the cooling load would be less than 2 tons given normal internal heat gains.
I guess the internal heat gains must be > 3,000 wattts.
I don't see that a true conclusion has been arrived at here.

Designer Dan
It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities